The present invention relates to medical instruments and methods and more particularly to an improved system and method in electroencephalography.
At the present time the most widely used instrument in electroencephalography is an analog system which amplifies and records the patient's brain waves. The system generally comprises a plurality of electrodes which are adapted to be connected to the patient's head, a set of amplifiers with each amplifier connected to a pair of electrodes, one of the pair generally being a reference electrode, and a multi-channel electrographic recording instrument connected to the amplifiers. Each channel of the recording instrument records a wavy line representing the patient's brain waves at the electrode corresponding to the channel input. For example, an ink pen of a galvanometer type recorder may record a wavy mark on a moving sheet of paper. The recorded set of waves is visually examined by a neurologist who, on the basis of his training and experience, will determine if there are any abnormalities. Neurologists, it is believed, would prefer to work with such recorded sets of waves for visual analysis since their parameters have been well documented and their major features known to them. Such major features include the frequency of the waves, their amplitude, the brain region (channel), waveform peculiarities, interhemispheric coherence, duration and envelope of recurring series of waves, voltage-frequency relationships and reactivity.
The present analog system suffers from a number of problems. The brain wave is a very faint electrical signal and it is easily drowned out by electrical signals arising from voluntary and involuntary muscle movement (muscle artifact). For example, the patient's eyelids may flutter and the electrical signals from such fluttering motion may obscure the patient's brain waves. It may be difficult, even for a highly trained neurologist, to visually distinguish a recording of muscle artifact from the recording of a patient's brain waves. In uncooperative patients, the bulk of the recorded material may consist of artifacts.
Another problem arises from patients' production of spikes and sharp waves, which may be the type of wave associated with epileptic activity. Although these sharp waves may be seen visually on a recording, it may be difficult at times to distinguish them from muscle artifact, sharp alpha waves and pulse rhythm. This distinction often depends on the context of the background activity in which they appear. In addition, the neurologist may wish to know the frequency of occurrence of such sharp waves over a period of time, and constructing an exhaustive inventory of such events is, by visual inspection, difficult and tedious.
In the inventor's U.S. Pat. No. 4,171,696, entitled "Prevention of Distortion of Brainwave Data Due To Eye Movement Or Other Artifacts," incorporated by reference herein, a system and method for limiting the adverse effects of muscle artifact is described. The system includes trans-orbital electrodes, a digital computer and a multi-channel recorder. Certain articles and patents relating to the problem of artifact compensation in electroencephalography are discussed in the U.S. Pat. No. 4,171,696, see column 1, line 46, through column 2, line 47.
The inventor, in his text "Neurometrics: Clinical Applications of Quantitative Electrophysiology, Functional Neuroscience, Vol. II" (Erlbaum Assoc. 1977), page 78, incorporated by reference herein, discusses reduction of the adverse effects of muscle artifact on brain wave detection in the context of a large digital computer based system. That system provided for the monitoring of eye movement and accelerometer channels, the accelerometer being attached to the patient's head. A threshold maximum amplitude value is set. First and second derivatives of the brainwaves are computed and evaluated relative to other threshold criteria. If artifact was detected, either the recording was marked or that portion rejected.
In U.S. Pat. No. 3,848,586 entitled "Signal Detection System," issued Nov. 19, 1974 to Suzuki et al, a signal detection system is described for use in an EEG instrument to reduce noise. The Suzuki et al patent points out that the prior art noise reduction method using analog low-pass filters were deficient because (i) the filter system attenuated high frequency components such as spike waves; and (ii) the filter system may product an artificial wave which becomes mixed with the brain wave signals. Suzuki et al sought to reduce noise due to muscle action (muscle artifact) by an analog-to-digital conversion of the brain waves, stretching the time axis, digital-to-analog conversion, and a recording using a pen recorder. In Suzuki et al waveforms witth a gradient steeper than a predetermined value (threshold C.sub.1) are removed by correcting them with a predetermined voltage (C.sub.2). The predetermined value selections are based on the patentee's theory that muscle artifact signals, compared to shapr waves, have a steeper gradient and a short duration.